<!doctype html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport"
        content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>通过 伪元素来实现</title>
  <style>
      .inner {
          width: 100px;
          background: red;
      }

      .inner::after {
          padding-bottom: 100%;
          display: block;
          content: '';
      }
  </style>
</head>
<body>
<div class="inner"></div>
</body>
</html>
